Frequency modulation receiver for overlapping signals



Patented Sept. 2, 1952 FREQUENCY MODULATION RECEIVER FOR I OVERLA PPINGSIGNALS Raymond M. Wilmette, Washington, D. 0., as signor to Padeveo,Inc., Washington, D. C., a

corporation of Delaware v Application January 24, 1950, Serial No.140,242;-

The present invention relates generally to apparatus and methods for theseparation of signals overlapping in frequency, and more particularly tomethods andjap'paratus for selectively separating two overlappingfrequency modulated earners.

It is a broad object'of the present invention to provide novelmethodsand apparatus for selectively separating two frequency modulatedsignals, which overlap in frequency, and which are of differentamplitudes. v v r It is another broad object of the invention to providea system 'for detecting modulation inherent on'a first frequencymodulated carrier, in the presence of :another'weaker overlappingfrequency modulated carrier, with substantial reduction'of interferencefrom the latter.

It is still another object of the present invention to provide circuitsfor selectively detecting frequency modulations of two superposed oroverlapping carriers, in response to signals obtained by sampling of thesuperposed carriers at selected time intervals.

It is another object of the present invention to provide circuits fordemodulating the stronger of two overlapping frequency modulatedcarriers, without interference from the weaker, when the 11 Claims. (01.25o 20) weaker has an amplitude approaching that of the stronger; by aprocessor timed sampling. 7

The above and still further objects, features and advantages ofjthepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,especiallylfwh'en taken in conjunction with the accompanying drawings,wherein: Figure 1 is a vector diagram of certain voltages occurring in-;the system' of' the present int n: e. 1 Figure z is asohematic'andblock circuit diagram of a specific embodiment of the invention;

Figure 3 is a schematic and block circuit diagram of a further specificembodiment of the invention; V

Figure 4 is a block diagram of a modification of the embodiment of theinvention illustrated in Figures 2 and 3 and Figure -5'is a blockdiagram of a, modification of the embodiment of the inventionillustrated i Figures 2 and 3.

Suppose two signals, which may be represented, in respect toinstantaneous values, by

where a 1 and q is theinstantaneous difierence in the frequencies of thesignals, and which may be positive or negative, at random, as timeproceeds.

The sum of the two signals is tors E1+E2, added together, andlabelled inrespect to the significance of thevarious parameters of Equations 1-3inclusive. In the discussion which follows it is taken that qt=0 when E2is collinear with and directed in the same sense as E1, i. e., at thecrests of the beat frequencies. It may be shown, then, that r arecalculated for certain values of at, the'Jol-s lowing table may beconstructed. Thistable gives the values of the quantities in terms of a,and of signals derivable from q and w by frequency discrimination whenand 1r radians and for two further values, 1. e., for the difference andthe sum of the quantities, taken at 0 and 1r radians.

As one mode of receiving the stronger of two overlapping frequencymodulated signals to the exclusion of the weaker, use may be made of thefact that when dt di and that this may be calculated to occur whenqtzn'i arc cos a. If, therefore, the radio fremay be measured by meansof a limiter and discriminator.

In accordance with the present invention, selected portions of the radiofrequency envelope of the superposed waves may be gated under thecontrol of the modulation wave at frequency q, to obtain the frequencyd(wt+a) (it for only the angles or values of qt specified in the table,columns (1), (2), (3). When so obtained simple voltage combinationsprovide the quantities specified in columns (4) and (5).

Additionally the quantity may be derived at the angles specified incolumns (1), (2), and (3), by multiplying the value of R at these anglesby the value derived for at these angles, in a conventional multiplier,the value of B being obtainable by suitably gating the resultofamplitude detecting the superposed waves. The values required forcolumns (4) and (5) may be derived by simple subtraction or addition ofvoltages, as required.

From the quantities specified in rows 1) and c, the signalscorresponding with frequencies w and (w+q) may be readily derived.

For example, the quantity specified in block 50 of the table is amultiple of the frequency w, and the quantity specified in block 40contains a quantity proportional to the frequency (w+q).

Since the frequency g may be readily derived .by a demodulationprocess,addition or substraction of frequencies in suitablev circuits may beemployed to obtain the quantities in the remaining blocks.

' the gating pulses.

quency at one or both of these points, in each cycle of beat frequency,is selected and applied to a discriminator, the result will be thestronger signal at frequency w. free from distortion due to the weaksignal.

Essentially, this type of operation may be accomplished approximately bysampling the two superposed signals E1 and E2 when which occurs when thebeat envelope has an amplitude near the mean, or unmodulated carrierlevel, of the stronger signal. This condition is substantially accuratewhen the ratio a, of the amplitudes of the signals, is small. When,however, the value of a is relatively large, it may be shown that anoptimum operating point exists when qt wi arc cos a, hereinaftersometimes referred to as "are cos a points, noting that 1r: arc cos itapproaches as a becomes smaller.

Essentially, then, demodulation of the stronger of two overlapping FMsignals is accomplished, to the exclusion of the Weaker, by sampling theoverlapping signals at selected times, or at selected points in the beatfrequency cycle thereof. and preferably when qi rrt arc cos 11. Samplingis preferably accomplished by generating gating pulses at the desiredtimes, and passing the overlapping carriers to a detector only inresponse to the gating pulses, or alternatively, detecting theoverlapping carriers and passing detected signal to a reproducer only inresponse to The gating "pulses themselves may be generated in a varietyof ways, which will appear as the description proceeds. In additionprovision is madefor smoothing out the signals generated in response tothe gating pulses to construct a true audio signal. The smoothingprocess may be carried out by a smoothing filter, if desired, but it isfound that electronic smoothing devices are preferable to filters, inpractice, because of the wide range of pulse frequencies which must behandled in the system. I have accordingly provided an electronicsmoothing circuit which is, per se, novel, for the purpose,

Referring now to Figure 2 of the accompanying drawings, the referencenumerals 38, 3!, 32, 33, 34, 35, denote respectively, an antenna, R. F.tuner, frequency converter, I. F. amplifier, limiter and discriminatorof a conventional FM receiver. It is assumed that the signals E1 and E2of Figure 1 are being simultaneously received, and that it is desired todemodulate the stronger signal without interference from the weaker, andthat the strength of: the weaker signal E2 may be sumciently great torender this impossible by conventional methods. The latter condition isnot a necessary one.

In order to demodulate the stronger signal without interference from theweaker use is made of the fact that the superposed wave R sin (wt+e) hasthe frequency to when qtzrriarc cos a but not otherwise, so that pulsesof frequency w, at'the input to a discriminator, or signals of amplitudecorresponding with w, may be made available by gating the wave R sin(HRH-u.) at times when qtzwriarc cos a; The problem remains then ofgenerating a gating wave at such times. This problem is solved in theembodiment of the invention illustrated in Figure 2 of the drawings by amanually controllable amplitude gating process. In Figure 3 a method ofselecting the arc cos a point is utilized which relies on the fact thatwhen two waves are beat together, one of which is frequency modulatedand the other not, very rapid rates of change of phase, which areequivalent to frequency shifts, occur when the relative phase of the twowaves passes in phase opposition. These frequency shifts are the greaterthe nearer in amplitude the two waves are, and have a sense dependent onthe sense in which the change in phase takes place or the algebraic signof the rate of change of phase. If the frequency modulated wave is acomposite wave obtained by beating two frequency modulated waves, suchas E1 and E2, Figure 1, and hence one which varies in amplitude, thefrequency shifts referred to will be of varying extent as the beat wavechanges in amplitude with respect to the amplitude of the unmodulatedwave, and of opposite directions as the amplitude increases anddecreases, respectively, and it will be found that the maximum shift maybe made to occur precisely at the arc cos a points, by properlyselecting the amplitude of output of the local oscillator.

The frequency shifts may be discriminated to provide waves of varyingamplitude, and the strongest of these waves selected by an amplitudeselection process and utilized as gating waves. M

Referring now again toFigure 2 of the drawings, the output ofdiscriminator 35 is applied to the input of a cathode follower 3 5, andthence applied to a gating tube comprising back-toback connected gatingtriode sections 31 and 38. The cathode 39 of section 31 and the anode 40of section 38 are connected directly to cathode load 4| of cathodefollower 36, and the anode 42 of section 31 together with the cathode 43of section 38 are connected directly to the control electrode 44 of acathode follower 45, having an output resistance 46 in its cathodecircuit and a smoothing condenser 47 connected between control electrode44 and ground.

The triode sections 3! and 38 are normally maintained nonconductive bymeans of a nega-. tive bias connected to the control electrode 48thereof from a source 49. This bias may be overcome by applying asuflicient positive voltage across resistance 58, in series betweencontrol electrode 48 and bias source 49. When the bias is overcomesufficiently the gating sections 31 and 38 pass alternating current,since, taken together they are bilaterally conductive by reason of theirback-to-back connection.

On-gating pulses for the back-to-back gating triode sections '31 and 38are developed as follows: I. F. output from amplifier 33, consisting ofwave 5|, R sin (wt+a), or E1+E2, and having thereon points a and boccurring at times qtzwiarc cos a, is applied to one primary winding 52of a transformer 53. To a further primary winding 54 of transformer 53is applied steady A.-C. wave from an oscillator 55 via an adjustableattenuator 58. The frequency of the output 51 of oscillator 55 isseparated from the frequencies w or w+ q by at least one channel.

The wave 51 is then added to the wave 51 in the secondary winding 58 oftransformer 53, and secondary winding 58 connected to the input of alimiter 59. The latter is in turn connected to a discriminator 68, andthe output of the'latter is applied across resistor 50, in such sense asto develop positive pulses thereacross. These pulses, as illustrated at6|, are of varying amplitude and sense, as wave 5| varies in amplitude,but reach maximum positive values 62 only when the amplitude of wave 55bears a predetermined relation to the ratio a between E2 and E1. Byproper adjustment of attenuator 55 the points selected may be at one ofthe qt==1riarc cos a points, and b, of wave 51.

The bias applied to the gating sections 37 and 38 is such that only thepositive peaks 62 gate the sections 3'! and 38 on, as indicated by thecut-off level associated with wave 6|. Hence there is applied tocondenser 41, via gating sections 31 and 38, only those portions of wave5! which occur at qt=vrarc cos a, in the form of discrete pulses. Thesepulses are stored in condenser 41 for the time between pulses, and maybe either positively going or negatively going in any order, since thecondenser 41 may either charge or discharge through gating sections 37,38, to extents depending on the polarity and amplitude of the last pulseapplied from cathode follower 36, as compared with the immediatelypreceding pulse.

The condenser 41 in conjunction with the gating sections 31, 38 providethen a smoothing circuit for the pulse output from cathode follower 36,storing the amplitude of each pulse until a succeeding pulse arrives,and then assuming a voltage corresponding with the amplitude of thelatter and storingthat until a further succeeding pulse arrives.

The voltage variations developed across condenser 41 are duplicatedacross output resistor 46 of cathode follower 35, and an audio signalcorresponding with the modulations in frequency of wave w may be derivedfrom across output resistance 46.

Referring now more specifically to'Figure 3 of the accompanyingdrawings, there is illustrated a modification of the system of Figure 12which utilizes some of the principles taught in Figure 2. Morespecifically, the system of Figure 3 employs amplitude gates, operatingon the envelope of the wave R sin (wt-l-a) to derive gating pulses attimes when qt=1ri arc cos'a. These gating pulses are applied to controla gating circuit which samples the discriminated superposed carriers attimes when qtzvri arc cos a, and applies the pulse samples to asmoothing circuit for construction of an audio wave.

Corresponding parts in Figures 2 and 3 are identified by the samenumerals of reference.

The wave R sin (wt+a), corresponding with E1+E2, possesses points a,which correspond with one value of qt:1ri arc cos a, and which areutilized to establish gating pulses. The wave 5! is applied to aconventional diode rectifier, "ill, in the load resistance H of which isgenerated a wave form 52 corresponding with the positive envelope ofwave 5!, and which appears as a wholly positive wave, with points aretained.

The wave i2 is amplified in beat amplifier 13, the latter accomplishinga phase inversion, and is applied across a load resistor is via acoupling condenser :5. The coupling condenser '15 removes the D.-C.component of the wave '52, so that its average value is zero. Thereappears across the load resistance is, then, an A.-C. voltage Hi,corresponding in wave form with wave "52, but phase inverted and havingan average value of zero. The points a now appear on the negativelygoing slopes of the spikes Tl, by reason of the phase inversion effectedby amplifier l3, rather than on the positively going slopes, as in waveform 12.

The ungrounded end or" resistor i is con nected to a cathode 80 of adiode 8!, the anode 82 of which is connected via a load resistor 83 toan adjustably positive point 84 of a bias source 85. If the cathode 86goes more positive than the anode 82 the diode 8! becomesnon-conductive. While the cathode 83 is negative with respect to theanode the diode 3i conducts in proportion to the negative voltage. Thediode 8i accordingly adjustably clips the positive peaks from the waveform T5, to provide a wave form 86, and the bias voltage provided bybias source 85 is so adjusted as to retain, by a narrow margin, thepoints a.

To the anode 32 of diode 3! is connected an anode 87 of a diode 58,having a cathode 89 connected via a load resistor 98 to a point 9! onbias source 85. The point 9| is negative with respect to point 84. Thevoltage at anode 8'! must at all times be equal to, or negative withrespect to, the voltage of point 86, according diode 8! isnon-conductive, or conducts. The bias voltage on cathode 89 is slightlynegative with respect to the bias voltage on anode 8'1. Hence the diode88 passes current only while the wave form 86 possesses voltage valuesbetween E84 and E 91, after which anode 81 goes more negative thancathode 39, and the diode becomes non-conductive.

Accordingly, the output voltage across load resistance 98 of diode 88corresponds with wave shape 92, and consists of rectangular pulsessuperposed on a steady D.-C. value, the trailing or negatively goingedges of the pulses occurring at times a, or encompassing the times a,since the trailing edges have finite slopes.

The wave 92 is applied via coupling condenser 53 to the controlelectrode 94 of a triode 85, and is transformed into a wave having zeroD.-C. component by virtue of the capacitive coupling as illustrated at96.

The cathode 97 of triode 95 is grounded, and the anode 98 is connectedin series with an inductance 98 to a source of 13+. The inductance 8, 99is shunted by a resistance I00, and 3+ is connected to control electrode9 via a high resistance lill, the resistance H11, in series with thegrid-cathode internal resistance of triode 95, providing a voltagedivider to establish a bias for grid 94 which is positive and whichmaintains the triode normally in current conductive condition. In thiscondition current flows in inductance 99 and energy is stored therein.

At the instant when the trailing or negatively going edges of the pulsesof wave form 96 occur, i. e., at times a, which, it will be recalledcorrespond with qt=wi arc cos a, the triode 95 is cut off, and remainscut oil until the wave form again goes positive.

When the triode 95 is cut off the energy in inductance 99 dischargesthrough resistance I60, and also through any inherent capacity which mayexist in the circuit.

The constants of the circuit are so selected that but a single energydischarge pulse occurs to accomplish the discharge, and the voltagegenerated is of such polarity that a positive voltage pulse I0 isapplied to line I02, by Lenzs law. This voltage gates triode sections 31and 38 on. The voltage pulses HH occur, then, when qt=1ri arc cos a, andthe output of the gating sections 37 and 38 corresponds with thatobtainable in the system of Figure 2, and is similarly treated.

Enclosed in dotted lines in Figures 2 and 3 are circuits which per sefunction to generate gating waves, in response to the overlapping wavesE1 and E2, or R sin (wt+a) when qt=1r arc cos a. The gating wavegenerator in Figure 2 may be identified by the letters GWGI, while thecorresponding generator in Figure 3 may be identified by the lettersGWGZ. In the systems of Figures 2 and 3 the gating waves provided bygating wave generators GWG! or GWGZ are applied to gate signalsoccurring beyond the discriminator. Gating may, alternatively, occurprior to the discriminator, as in the I. F. or R. F. circuits, andpreferably the former. Modification of the system of Figure 2 isprovided accordingly, in Figure 4, wherein gating occurs in the I. F.channel. Obviously, a similar modification of the system of Figure 3 maybe resorted to, if desired.

As a further modification of the invention, gating waves are providedwhen 1r qt-i by means of a gating wave generator illustrated in Figure 5of the drawings, for the reason that a gating wave generator of thischaracter is especially simple. Essentially, such a gating wavegenerator utilizes the fact that qt=-i arc cos 11, approximately, whenthe amplitude detected envelope of R sin (wt-ta), with D.-C. componentremoved, passes through zero, and when a is small. This embodiment ofthe invention is useful primarily when a is small but may be readilymodified to render the circuit useful for a wide range of values of a.

The gating wave generator of Figure 5 may be made more useful by addingto the detected envelope of R sin (wt+a.) with D.-C. component removed(see Figure 6), an adjustable D.-C. component, which shifts theeffective cross over point of the envelope of R sin (wt-l-a), on thepoint of polarity reversal of R sin (wt-ta), with D.-C. component, to atime which may be made to correspond with qt=1ri arc cos a, so that thesystem or Figure'5 provides-'an additional gating" wave generator GWG3;suppleme'ntaryto gating wave generators GWGI and GWG2, and one which maybe utilized in the systems of Figures 2, 3 and 4.

Since, in the gating systems of the embodiments of my inventionillustrated in Figures 2- and 3, the gating waves] applied to gate onthe triode sections '31 and 38 must be of constant amplitude, if theamplitude of the'gating pulses is "not to be a reeman the output signalprovided by cathodefollower 45,,itm'ay prove desirable to clip ;the.gatin'g=pulses to :constant amplitude, as byr'neans of a clipper stagebetween discriminator sc ne resistance Figure 2, and by a-clipper'stageinserted in-the lead I 02 of Figure 3." Additionally, it will berealized that the output derivable across output resistance 46, inFigures 2 and 3, will contain a D.-C. component due'to the gatingpulses. This D.-C. component may be efiectively eliminated by applyingoutputvoltage, derived across resistance 46 to its utilization circuit,via a transformer or condenser coupling.

Referring more specifically to Figure 4 of the drawings, the system willbe seen to duplicate the systems of Figures 2 and 3 except that gatingsections 31 and 38 are no longer utilized for gating E1 and E2, butgating is accomplished in the I. F. amplifier 33. or between I. F. andlimiter 34, by means a'normally closedgate G, which is turned on inresponse to' gating waves generated by gating wave generatorI-I, whichmay correspond with GWGI .or GWGZ' 1 7 The output of the discriminator35 is applied to cathode follower 36, and is in the form of discretepulses. These pulses, as taken from the load resistance 4| of cathodefollower 36, are smoothed by passing the pulses through backto-backgating triode sections 31, 38, to storage condenser 41. In order toenable storage condenser 41 to retain its charge between pulses, thedouble triodes 31, 38 are normally biased off by bias source I I0, andrendered conductive in response to each pulse applied thereto. Thelatter function is accomplished by passing the pulses, as they appearacross load resistance 4| of cathode follower 36, through an amplitudeclipper, which provides at its output fixed amplitude pulses III. Theselatter are of sufficient amplitude to overcome the bias supplied bysource Hi3, and to turn on the gating triode sections'3'I and 38 duringthe pulses. The pulses passed by gating triode sections 31 and 38 arethen utilized to charge condenser 41, which acts as a smoothing mediumsince it cannot discharge between pulses.

In the system of Figure 5 the gating wave generator is a pulser I whichoperates to generate pulses I2I when an A.-C. wave I22 passes throughzero. Such pulsers are conventional in the art. The A.-C. wave I22 isprovided by amplitude detecting the output of I. F. amplifier 33 indetector I23 and passing the detected result through a condenser I24 toremove the D.-C. component of the detected wave. The pulses occur thenwhen the detected beat envelope I22 passes through zero, and thissufiiciently approximates the arc cos a point when a is small, forpractical purposes. Adjustment of the pulsing point may be attained byadding to the input of the pulser I20 a D.-C. bias, by means of avariable bias source I25, effectively to shift the times when A.-C. WaveI22 passes through zero value, and

- in respect to any of the understood by those thereby to shift are sassof pulses I2Ij to anew for operation for a range of value Itwill beclear thatfurther; var or in general arrangement in I again, e esortedto;

of my mvention; i

pended claims. 'Specificall'yja l umberfof methods for generatingpul'ses*atpr'edtermined amplitude points of a wave form are known, and may beemployed instead of those herein illustrated and described as'GWGI, GWGZand GWG3. Particular reference is made to" chapter 9 of RadiationLaboratory series. #19; entiti s "Waveforms," published 111 1949 byMcGraw Hill B001; 00., -Inc., a for detailsof gating wave "genera torssuitable foruse in the pre'sent system'jfflfi What I claim and'desire'toPatent of the United States a na e.aaaaeaee er wiser,

two Overlapping e uency .g lmdl y fid lav. and Ez having an am'plituderao" and'an instantaneous.frequency differencegq, are

- osaid. ea i Vr a eafar i tivi a m a overlapping frequency modulatedwaves energy pulses corresponding in amplitude with said modulation, andmeans responsive to said energy pulses for constructing said modulation.

2. In a frequency modulation receiver wherein two overlapping frequencymodulated waves, E1 and E2, having an amplitude ratio and aninstantaneous frequency difference q, are received, and wherein it isdesired to obtain modulation of the stronger of said waves to thesubstantial exclusion of the weaker, means for generating pulse gatingvoltages substantially at times when qt=1r-+ arc cos a, means forlimiting and discriminating said overlapping frequency modulated waves,to obtain modulation corresponding to said overlapping frequencymodulated waves. means responsive to said pulse gating voltages forselecting, from said last mentioned modulation, pulses corresponding intime with said pulse gating voltages, and means responsive to said lastmentioned pulses for constructing said modulation of the stronger ofsaid waves.

3. Thecombination in accordance with claim 2 wherein the means forgenerating the gating voltages comprises means for adding to said twooverlapping frequency modulated waves a wave of fixed frequency andadjustable amplitude to provide a sensing wave, means for frequencydiscriminating said sensing wave to provide pulses of varying amplitude,and means for selecting from said pulses of varying amplitude thosepulses of amplitude exceeding a predetermined level.

4. The combination in accordance with claim 3 wherein said adjustableamplitude is selected to equal substantially the amplitude of saidsecure by Letters 11- overlapping frequency modulated waves when It-=11:arc cos a a 5. The com ination in accordance with claim 2 wherein themeans for generating the. gating voltages comprises means for generatingpulses having abrupt changes in level when qt ri arc cos' 'a, and, meansresponsive to said abrupt changes in level for generating said gatingpulses.

6. In a frequency modulation receiver wherein two overlapping frequencymodulated waves E1+E2 are. received, where and where the beat frequencybetween the waves is q, means: for abstracting, from said waves E +E2modulation of El) to the substantial exclusion of modulation of E2comprising means for generating timed signals in accordance with apredetermined value of qt at least adjacent r: are cos a, and means for;providing modulation deriving from frequency modulation of saidOverlapping frequency modulated waves E1+E2 only duringsaidtimedsignals.

7. The combination in accordance with claim 1 wherein said frequencymodulation receiver is a superheterodyne receiver and includes anintermediate frequency amplifier and wherein said means responsive tosaid gating waves includes said intermediate amplifier.

8'. The combination in accordance with claim 1 wherein said receiverincludes an, audio amplifier} and wherein said means responsive to saidgating voltages includes said audio amplifier.

9. In a frequency modulation receiver wherein from saidoverlapping'frequeney modulated waves recurrent wave energy pulseshaving a wave energy characteristic representative of said modulation ofthe: stronger of said waves. and means responsive to said wave energypulses for con.- structing said modulation.

10. The combination in accordance with claim 9 wherein saidcharacteristic is frequency.

11. The combination in accordance with claim 9 wherein saidcharacteristic is amplitude.

RAYMOND M. WILMOTTE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Namev Date 2,194,292 Bligh Mar. 19, 19402,295,207 Gabrilovitch Sept. 8, 1942 2,361,437 Trevor Oct. 31, 19442,386,528 Wilmotte Oct. 9, 1945 2,467,486 Krurnhansl et a1. Apr. 19,1949

